The present invention relates to methods and apparatus for manufacturing paper, and in particular, paper manufactured which uses a substantial portion of recycled stock such as old corrugated container (OCC) fiber, de-inking grades and mixed paper.
It is desirable to increase the percentage of recycled fiber, such as OCC fiber, that is used in the manufacture of paper such as linerboard and other kinds of paper such as tissue paper. However, increasing the percentage of OCC fiber for example, causes the appearance of waxy specs in the linerboard, and such waxy specs render the linerboard quality unacceptable in certain container applications. The waxy specs causing this visual quality problem in the final linerboard product comprise melted or fused paraffin wax, polyethylenes, hot melt, polypropylene film, polyethylene film, foam plastic such as styrofoam and urethane foam, latex glue residue, fused polystyrene, etc. Most of these constituents are organic materials, which can be synthetic or naturally occurring, that are solids at room temperature, but which flow at higher temperatures. Because of this transition to soft highly pliable solids or high viscosity liquids at temperatures in the 120.degree. F. to 180.degree. F. range, it is difficult to screen out those constituents. Moreover, since the bulk specific gravity of these constituents are frequently in the same range as process water, i.e., 0.95 to 1.00, they are difficult to separate by centrifugal means.
In an attempt to improve the quality of the final paper product, paper mills which recycle OCC fiber have used systems known as asphalt dispersion systems in preparing the stock. A typical paper mill using OCC fiber begins the process by dumping OCC into a pulper and adding water until the mixture is a consistency of about 4% solids by weight. The pulper has rotating mechanical elements which break down the solids into a slurry consisting of paper pulp and contaminants. Typically, the contaminants include bulk contaminants such as larger pieces of metal and glass, nails, staples, beverage cans and bottles. There are also smaller particulate contaminants consisting of inorganic matter such as sand and various organic materials such as waxes, polyethylene films, polypropylene, glue, styrofoam, etc. Many of the smaller particulate contaminants are sticky substances which will tend to attach to one another and agglomerate into larger particles of sticky substances. The stream of paper stock leaving the pulper with a 4% consistency typically was directed to a holding tank for purposes of providing some means of controlling the flow of stock through the mill. The 4% consistency contaminated stock then is passed through a rough screen which filters out bulk contaminants such as glass bottles, cans, chunks of metal and other coarse debris. Then the contaminated stock is passed through two kinds of thickeners which produce a stock stream of approximately 30% consistency by weight. An extractor screw is one kind of thickener which removes water from the stock stream as it is moved through the length of a vessel containing a slowly rotating screw. A second kind of thickener is a press, which squeezes additional water from the stock stream. The water which is removed by the thickeners to go from a 4% consistency contaminated stock stream to a 30% consistency contaminated stock stream is typically pumped back to the pulper. The 30% consistency contaminated stock stream next is fed into a digester, which is a massive pressure vessel surrounding a large metal screw which slowly rotates at around 100 rpm and carries the stock stream forward through the digester. The digester also receives steam which heats the contaminated stock to around 300.degree. F. and pressurizes it as it moves through the digester. The digester sometimes is referred to as a "steaming tube". Upon exiting the digester, the contaminants have been dispersed throughout the pulp by the heating action of the steam and the pressurizing action of the steam and the screw. This dispersed contaminant stock stream is converted from 30% consistency to 4% consistency by pumping water into it. Then the 4% consistency dispersed contaminant stock stream is passed through a refiner, which is a massive pressure vessel having two rotating interfacing plates that act as a mill. Typically, the plates of the refiner rotate at approximately 1600 rpm. The stock is fed to the center of the two rotating elements, which have radially extending ribs on their interfacing surfaces, and the relative movement between the rotating plates mechanically works the 4% consistency dispersed contaminant stock stream and moves it from the center of the plates to the periphery of the plates and out of the refiner to another holding tank. The dispersed contaminant stock in this holding tank is ready for processing into paper, as by a fourdrinier machine. Typically, this stock is used to feed the secondary head box of a fourdrinier machine or the liner vats of a combination paperboard machine.
Thus, an asphalt dispersion system treats the entire stream of the accept feed which goes to the paper making machine. Such full steam treatment by the asphalt dispersion system causes hemicellulose extraction from the entire stream. This is because the asphalt dispersion systems are characterized by high heat over a long period of time. Such hemicellulose extraction reduces fiber strength and bonding and thereby reduces paper board strength.
In addition to having the disadvantage of weakening the strength of the final paper product, the asphalt dispersion system also has the disadvantage of being very costly to implement and operate. The thickeners and the digester are very expensive pieces of equipment. They all must be large enough and sturdy enough to accept the entire stream of contaminated stock to be used by the paper mill and to subject it to pressures of the stream and moving parts therein. Moreover, heavy duty water pumps and chests are required at different sites to handle the consistency changes of the stream as required by the asphalt dispersion treatment. A typical asphalt dispersion system costs in the neighborhood of $300,000 to $500,000, depending upon the sophistication of the thickeners. While the less sophisticated thickeners tend to keep the cost in the $300,000 end of the range, they also require significantly more maintenance. The operating cost for a typical asphalt dispersion system is approximately $13 per ton of pulp. A significant amount of steam is required to treat the entire stream of stock, and the cost of providing this steam is significant. The cost of maintaining the thickeners also is significant, especially in the case of the older and less sophisticated thickener equipment which still is used by many producers of recycled stock.
These costs have prompted some paper mills to eliminate the asphalt dispersion system in favor of a plurality of screening mechanisms which pass the contaminated stock stream through a series of screens. The later screens have smaller openings than the earlier screens, and the later screens also have openings of a different shape such as slotted versus circular. After this screening, the stock stream is passed through a refiner. Since the screens are less costly than the thickeners and digester, and since the need for steam has been eliminated, this alternative system is a less costly alternative to the asphalt dispersion system. Whatever small particulate impurities remain after screening are dispersed in the refiner, but the visual quality of the final product is not as acceptable as the visual quality of the final product from the pulp processed through the asphalt dispersion system.
Chemical treatment of the entire stock stream has been used in conjunction with both the asphalt dispersion system and the screening system. The chemicals added to the stock stream typically are combinations of organic dispersants, wetting agents and inorganic materials such as talc. These chemicals promote dispersion of the organic contaminants and break up particles of such contaminants and hold them in suspension so that they do not agglomerate and form noticeable blotches in the final paper product. Such blotches render the quality of the final product unacceptable to the degree that the blotches are prevalent. The amount of chemicals needed to treat the entire stock stream constitutes a significant operating expense.